The present invention relates to an ink composition for a meltable ink usable in a printing device in which ink drops are ejected from ink ducts. The present invention comprises an agent which reversibly cross-links the fluid ink. The present invention also relates to a method of printing a substrate with such an ink composition.
Inks of this kind, which are solid at room temperature and liquid at an elevated temperature are known from U.S. Pat. No. 5,380,769. These inks are reactive compositions which contain at least two components, i.e. a basic ink component and a reagent, the components being transferred to a receiving medium separately from one another. Exposure of the basic ink component to the reagent results in the formation of polymers which form a network in the fluid ink by means of reversible bonds.
An important disadvantage of an ink composition of this kind is that it consists of two separate components which have to be transferred successively to a receiving medium. This makes the printing device complex and the production and processing of the ink expensive.
The object of the present invention is to obviate these disadvantages. To this end, an ink composition has been invented in which the agent which reversibly cross-links the fluid ink comprises a gelling agent. A gelling agent is capable, as such, to thicken a liquid by forming a three-dimensional structure therein. The liquid thus passes over into the form of a gel. A gelling agent can consist, inter alia, of high and low molecular compounds, a mixture of compounds, or of discrete particles. The molecules or particles of the gelling agent so interact with one another in the gel that a network is formed in the liquid. During this network formation, it is not necessary for the molecules or particles of which the gelling agent consists to be actually chemically bonded or have physical contact. All that is required is that they should have a physical interaction such as to result in a reinforcing effect in the liquid. As a result the viscosity of the liquid increases without it passing over into a solid phase.
Typical gelling agents are high-molecular, elongated molecules which form an elastic network in a medium, the interstices of the network being filled with the medium which can be in a liquid or a solid state. If the medium in the interstices is in a liquid state, a gel arises which has some fluid-like properties, such as the property that molecules can diffuse relatively easily into the continuous liquid matrix and some solid-like properties such as the property that the gel can withstand a certain shear stress without deformation occurring, before the gel starts to flow like a liquid. When the liquid in the interstices of the network solidifies, the gel passes over to the solid state.
The ink composition is preferably a gel at a temperature equal to or higher than room temperature. In this way a network can be formed by a printing ink on a receiving medium at ambient or elevated temperatures. In another preferred embodiment, the ink composition is a gel at a first temperature and a sol at a second temperature higher than the first temperature. A sol is a low viscosity, homogeneous liquid, which may contain colloidal particles. The ink is cross-linked in such a fashion that the links are broken when the temperature is elevated above the second temperature, also called the gel-transition temperature. This is a very important advantage for the inks of the present invention because in this way it can be assured that the inks have a low viscosity at the jetting temperature. A low viscosity ameliorates the jetting characteristics of an ink jet ink.
The use of gelling agents is of course known from liquid inks (aqueous and solvent inks), but these gelling agents cannot be directly used in a meltable ink. It is in fact impossible to select a molecule or particle that can cause a liquid to pass over into a gel. Whether a specific compound has the property of being able to cause a liquid to pass over into the form of a gel also depends on the properties of the liquid. Moreover, in the case of meltable inks it is difficult, accurately, to co-ordinate the different solubilities of the different components in such a manner as to form a homogenous melt at the temperature at which ink drops are ejected from the ink ducts, and a specific solidification and crystallisation behaviour is exhibited on cooling of the melted ink. This makes the development of meltable inks very complex.
Known gelling agents such as carragenan, laminarane, pectin and gums such as gum arabic, xanthane and guar gums, are high-molecular polymers. Since meltable inks of themselves are already more viscous than liquid inks at the temperature at which the ink drops are ejected from the ink ducts, the addition of a small quantity of such high-molecular gelling agents can lead to an unacceptably high viscosity in the melted state. This means that there is an adverse effect on the jet properties of the ink. Oligomer gelling agents, i.e. gelling agents with a molecular weight of less than 10,000, which are less common are therefore preferably used, so that a higher percentage of gelling agent can be added to the ink composition without having an adverse effect on the melt viscosity of the ink composition. In a further preferred embodiment, low-molecular gelling agents are used, i.e. gelling agents with a molecular weight of less than 1,000.
The fact that oligomers and low-molecular weight compounds can have gelling properties despite their relatively low molecular weight can be explained as follows. In the case of oligomers and low-molecular gelling agents, the molecules separate from the melted ink on an adequate reduction of the temperature, and form long compound chains via mutual non-covalent interactions, said chains behaving in accordance with the high-molecular polymers in the previously mentioned gelling agents. The compound chains can form a network which causes the melted ink composition to pass over into a gel. Since the molecules of the gelling agent form a network structure, concentrations of the gelling agent as low as 25% can be sufficient to gel the liquid ink composition. When the gel is heated up, the interactions between the molecules of the gelling agent are interrupted and a sol re-forms. A supplementary advantage of the use of oligomers and low molecular weight gelling agents is that the gel-sol transition takes place relatively quickly. Because of this transition, it is only necessary to break the relatively weak non-covalent bonds between the compound molecules of the polymer chains. In addition, small molecules will be mixed homogeneously in the melted ink matrix more rapidly. This is an important advantage because a meltable ink frequently has to be brought into the melted state quickly, e.g. when the printing machine is started up and the user wants to print an image immediately. For use in a hot melt ink, a gelling agent is preferably used which has amphiphilic properties, i.e. partly polar and partly apolar properties. An example is a gelling agent with a straight alkane backbone and some additional polar groups. Because of the amphiphilic properties the combining of the gelling agent with various hot melt ink compositions is simplified. Gelling agents having an optimal function according to the present invention only need to be present in a quantity of less than 10% based on the total weight of the ink composition. Finding such gelling agents is difficult but has the advantage that no adverse effects on the viscosity of the melted ink and other physical properties, such as the melt-temperature and the adhesive and mechanical properties of the solidified ink, nor the solubility properties of the liquid ink are expected as a result. In a further preferred embodiment, the gelling agent constitutes less than 5% of the total ink composition.
To ensure that the ink does not gel when it is situated in an in duct prior to the ejection of an ink duct from said duct, the ink is preferably a low-viscosity sol at the temperature at which the ink drops are ejected from the ink ducts (the xe2x80x9ctemperature of usexe2x80x9d, typically 60xc2x0 C. to 160xc2x0 C.) and a gel at a lower temperature differing by at least 10xc2x0 C. from the temperature of use. To prevent an ink drop from gelling too quickly when it cools on a substrate, which could result in poor adhesion to the substrate, the temperature at which the ink composition passes from a sol to a gel is preferably at least 20xc2x0 C. different from the temperature of use. After the ink has gelled on a substrate, it will cool further, and finally the meltable fraction of the ink composition situated in the interstitial space of the network will pass over to the solid state. In this way an ink drop obtains the strength required to offer sufficient resistance to mechanical deformation resulting, for example, from gumming, scratching and folding of the substrate.
U.S. Pat. No. 5,902,841 discloses hot melt ink jet inks which contain hydroxy-functional fatty-acids. These inks transfer into a metastable gel-phase when cooled from a liquid phase at elevated temperatures to a solid phase at ambient temperatures. However, the metastable gel-phase is a solid state gel in which the interstices of the network are filled with solidified ink.
In U.S. Pat. No. 5,989,325, published after the priority date of the present application, a non-aqueous ink composition comprising a hydrophobic gelling agent is disclosed. This ink composition also transfers from a liquid state at elevated temperatures to a solid, gelled state at lower temperatures.
Therefore, these inks differ from the ink composition of the present invention which relates to the gelling or cross-linking of an ink in a fluid state.
A meltable ink composition containing a gelling agent has a number of surprising advantages compared with the known meltable inks. The most important advantage is that an ink composition according to the present invention gives very good printed results on porous substrates such as paper. Even in the case of relatively hot substrates there is no unacceptable running of the ink drops, which might result in printed lines becoming unsharp (xe2x80x9cfeatheringxe2x80x9d), colors running into one another (xe2x80x9ccolor bleedxe2x80x9d) or ink at the back of the substrate (xe2x80x9cbleedingxe2x80x9d). If the ink already passes over into the gel state at a relatively high temperature during the cooling process, this has the result that the drops become relatively immobile because of the increased viscosity. This prevents the ink drops from flowing uncontrollably into the paper. It is essential however that the ink, in spite of its gelled state, still has sufficient fluid-like properties. These properties are present since the ink in the interstices of the network is still in a melted, fluid state. This way, a gelled ink drop can readily penetrate in the paper in order to prevent setting of the ink on the upper surface of the paper.
Inks without gelling agents continue to behave as a liquid for as long as the temperature of a drop is above the solidification point. In the case of hot substrates in particular, ink drops frequently cool too slowly. As a result, ink drops with known ink compositions remain fluid for too long so that a printed pattern loses its sharpness. In the known inks, this is prevented, inter alia, by selecting an ink composition with a higher solidification point. However, a composition of this kind has several disadvantages. Firstly, a higher solidification point normally goes together with a higher melting point, so that a printing device has to be heated to a higher temperature, and this subjects the printing device construction to much more stringent requirements. Secondly, an ink drop with a higher solidification point tends to set on the upper surface of a receiving medium. In this way the ink dot cannot withstand mechanical impact and will, for example, be subjected to smearing, a phenomenon that is very common in conjunction with hot melt inks.
A concomitant advantage of an ink composition according to the present invention is that the running of the ink drops is no longer dependent on the solidification behaviour of the other components present in the ink composition. In an ink composition according to the present invention, running is determined by the temperature at which the ink passes over into the gel state and not by the temperature at which the ink passes over to the solid state (for example, because a crystalline or amorphous component in the ink composition solidifies). This means that the other materials forming part of the ink composition can be selected from materials which solidify more slowly or more quickly without this having a perceptible adverse effect on the running of the ink drops. A slower solidification may be desired, for example, in order to give an ink a stronger interaction with the sizings of a substrate. A faster solidification may be desired to give the printed substrate the required resistance to gumming, scratching and folding sooner. The network of gelling agent molecules can also serve as a seed for the solidification of the other materials. If, for example, a crystalline material is selected as an important constituent of an ink composition, the addition of a gelling agent can result in a more microcrystalline matrix which delivers smooth matt-gloss prints. With such an ink it is possible to use a less pure crystalline material or to add more amorphous material to the ink composition, for example, to increase the dissolving power for dyes, if the crystallization of the crystalline material in the ink matrix is sufficiently stimulated. A faster crystallization of an ink constituent could be advantageous because than the ink composition becomes hard relatively soon after cooling of the ink composition to room temperature. This is not only advantageous in the printing of substrates (printed substrates will be resistant against mechanical impact, such as gumming, scratching and folding, relatively soon after printing), but also advantageous in the manufacturing of ink pills out of molten ink (pills solidying in molds will become sufficiently hard relatively fast so that they can be released from their molds are a short while to be handled further). Finally, by enhancing crystallization a disturbing after- or re-crystallization, which could become visible as a white haze in a printed image, can be prevented.
It is possible to apply a mixture of gelling agents in an ink composition. This way the gel-forming process can be adjusted precisely and next to that, the solidifying of the other ink constituents, in particular the crystallization of a crystalline constituent can be stimulated in any desired way. With pigmented inks it has been found that the use of a gelling agent gives a better distribution of the pigment, so that printed layers of such an ink are more uniform than if no gelling agent is used.
Moreover, the use of a gelling agent has also been found to provide advantages when smooth non-porous substances are used as a medium. Coagulation of ink drops printed in each other""s vicinity has been found to be greatly suppressed. This phenomenon is probably also related to the reduced mobility of gelled ink drops.
A concomitant advantage of an ink composition occurs when the ink is used for multi-layer prints, as is conventional with four-color printing. Ideally, a following drop is printed on a preceding drop when the preceding drop has not yet completely solidified. This results in good adhesion and color mixing. This means that inks are preferably selected which remain fluid on a substrate for a fairly long time. However, if ink drops remain fluid too long, unwanted running occurs, so that printed images become unsharp. By using an ink composition according to the present invention it is possible to prevent unwanted running of an ink drop that has not yet set, and yet good adhesion occurs with a following ink drop. Thus, a gelled ink drop behaves like a fluid ink drop in so far as adhesion and color mixing is concerned, but does not suffer from adverse feathering.
Finally, it has been found that an ink composition according to the present invention is very suitable for a printing device provided with an after-treatment device, particularly when the after-treatment is combined with heat transfer to the ink drops: what is known as thermal after-treatment. If printed layers of known inks are after-treated in order to give the ink drops better interaction with the paper, e.g. to obtain better gumming, scratching and folding resistance, or in order to improve the uniformity of the printed image, for example to provide a more uniform gloss, this quickly results in unwanted running of the ink because the ink frequently has to be heated to above its melting point and thus, because of the lower viscosity, can enter the paper uncontrollably. When an ink composition according to the invention is used in combination with an after-treatment device, the temperature of the ink drops being increased so that they pass over from the solid state to a gelled state, it has been found that the gumming, scratch and folding resistance of the treated substrates increases greatly without accompanying uncontrolled running of the ink. Thus, a gelled ink drop has sufficient fluid properties in order to migrate into a substrate further, but the cohesion in the ink drop is so great that uncontrolled running of the ink drop is prevented.